ERT imaging of root water uptake · 2021. 3. 12. · ERT imaging of root water uptake Chris Watts...
Transcript of ERT imaging of root water uptake · 2021. 3. 12. · ERT imaging of root water uptake Chris Watts...
ERT imaging of root water uptake
Chris WattsRothamsted Research
Andrew Binley, Lakam MejusLancaster Environment Centre
Lancaster University
Email: [email protected]
New Technologies in Soil MeasurementsIAgrE Soil & Water Group
Cranfield University, Bedfordshire. 18-01-13
Trial the use of electrical resistivity tomography (ERT)for mapping spatial and temporal variation in soilmoisture, ultimately for evaluating the extent of rootwater uptake
Objectives
Electrical resistivity
Basics
Resistivity (r) in Wm or Ohm-m (intrinsic property)
Resistance, R over length, L
r= R A
L
Cross sectional area, A
Resistivity (r) = 1 / conductivity (s)
r=1/s 1 Wm 1 S/m
For soil resistivity depends on physical andchemical properties:• degree of saturation (water content)• electrical resistivity of fluid (solute
concentration)• texture (particle size distribution, mineralogy)• arrangement of voids (porosity, pore size
distribution and pore connectivity)• temperature.
Soil Resistivity
Resistivity/conductivity
Archie’s empirical law (Archie, 1942) is themost widely used.
nw
mw S
mwF
Formation factor: Cementation index:
35.1 m(typically)
Saturation index:
23.1 n(typically)
Fluid conductivity Porosity Saturation
Hydrogeophysical relationships
Valid for medium or coarse-grained soils.
If clay fraction is significant then we must also account forsurface conductivity
S
BQS v
wnw
m
saturation
Pore water conductivityparticle
geometry
porosity
Texture/particle size
Hydrogeophysical relationships
vQ
B Equivalent ionic conductance of the clay exchange cations
Effective clay content
But note that changes in moisture content will be easierto interpret – if fluid conductivity is constant (ortemperature compensated)
S
BQS v
wnw
m
saturation
Pore water conductivityparticle
geometry
porosity
Texture/particle size
Hydrogeophysical relationships
Aim: Image underground soil moisture patterns bothspatially and temporally using ERT.Principle: Transmit current, I through two electrodesand measure a voltage with two other electrodes.Apparent resistivity; ρ=k V/I, where k is a function ofelectrode spacing/geometry.Resistivity pseudo section; contour plot of apparentresistivity data, using electrode distance and pseudo-depth parameter.True resistivity section; contour plot of resistivitydistribution obtained through the inversion ofmeasured data (using non-linear parameter fittingscheme).
Resistivity imaging
Current is injected between C+ and C-The voltage difference between P+ and P- is measured
The voltage difference is a function of the currentinjected and the resistivity beneath the electrode array
C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-
Electrical resistivity tomography (ERT)provides an assessment of lateral andvertical structure
Electrical resistivity tomography (ERT)
We can change the electrode spacing and position inorder to ‘sense’ the ground at different depths
Electrical resistivity tomography (ERT)
Distance (m)
Electrode
Surv
ey
leve
l
1
3
5
7
C+P+
P-C- P+ P- C-C+
5 10 15 20 25 30 35 40 45
We then need to carry out data inversion in order todetermine the distribution of resistivities that areconsistent with the data
Electrical resistivity tomography (ERT)
0 5 10 15 20 25 30 35 40 45-8
-6
-4
-2
0
Ele
vation
(m)
Electrode
100 Ohm-m10 Ohm-m10 Ohm-m
True model
Inverted model
0 5 10 15 20 25 30 35 40 45-8
-6
-4
-2
0
1009080706050403020
Resistivity (Ohm-m)
Distance (m)
Distance (m)
Ele
vation
(m)
• Woburn, Sandy loam soil (drought prone)• 12 wheat varieties x 2 levels of N (100 kg & 200 kg) x 2 water
strategies (rain-fed (water stressed) & well-watered (irrigationtapes alternate rows)
• Plots 10 m x 1.8 m; 3 (reps) x 12 x 2 x 2 = 144 plots• Two ERT arrays span 12 wheat plots (irrigated) & 12 un-
irrigated and remain in-situ from February to mid August
Pilot study: Wheat Drought Experiment
Electrical imaging at the Woburn site
2011 Pilot Study results
19-Apr-2011SYSCAL Pro Switch is aelectrical resistivity combinedtransmitter, receiver andswitching unit
Measurements also done on irrigated strips
19-Apr-2011
Electrical imaging at the Woburn site
2011 results
Electrical imaging at the Woburn site
Rain fed (unirrigated) results
log10 resistivity, in Wm
Wheat plot Drying underplant growth?
19-April-2011
2011 results (static measurements)
25 Wm40 mS/m
1000 Wm1 mS/m
160 Wm6 mS/m
Lowyield wheat
Highyield wheat
Comparison with irrigated profile results
log10 resistivity, in Wm
19-April-2011
Irrigated
Rain fed
2011 results (static measurements)
log10 resistivity, in Wm19-April-2011
Comparisonwith ProfileProbe pointmeasurementsof moisturecontent
2011 results (static measurements)
log10 resistivity, in Wm19-April-2011
log10 resistivity, in Wm
In some cases thecomparison withpointmeasurements isstraightforward
2011 results (static measurements)
log10 resistivity, in Wm19-April-2011
But not in allcases.
What do wecompareagainst here? log10 resistivity, in Wm
2011 results (static measurements)
2011 results (dynamic measurements)
Change in resistivity (%)
Change in resistivity from 19-April-2011
13-May-2011
14-July-2011
08-August-2011
Rain-fed
2011 results (dynamic measurements)
Need to compensatefor temperature?
2011 results (dynamic measurements)
14-July-2011
Change in resistivity (%)
Change in moisture contentfrom 19-April-2011
Change from19-April to 14-July
Current plans
Following promising results of water extraction patternsunder experimental wheat crop, we aim to:
• Determine relationships between electrical conductivityand soil water content for test sites.
• Carry our ERT surveys on contrasting soils/plants(monitoring program) and assess ability to estimatemoisture content from electrical conductivity.
• Apply EMI at same sites/conditions and develop ameasurement protocol for its use in mapping soil watercontent variation at the field scale, and over time.
Transmitter creates primaryelectromagnetic field
GPS tracks location
ReceiverTransmitter
Primary field
Secondary fieldEddy currents
Conductor
Receiver measuressecondary field createdin the ground (which is
a function of theelectrical conductivity
of the ground)
Electromagnetic Induction (EMI)
Data logger
Orientation of the coils alsoallows us to change thedepth of investigation
Electromagnetic Induction (EMI)
Callegary et al.(2007)
Electromagnetic Induction (EMI)
Traditionally EMI instrumentshave been deployed using onedepth of investigation – useful forreconnaissance type surveys.
New instrumentation providesmultiple coil separations – givingmultiple depths at one location.
For example, the GF InstrumentsCMD Mini Explorer has coils at1.18m, 0.71m and 0.32m in oneinstrument.
Electromagnetic Induction (EMI)
This gives us 6 possible depths of investigation
0.5m1.0m1.8m
0.25m0.5m0.9m
EMI at the Woburn site - Initial trials June 2012
Example results
Example results
EMI at the Woburn site - Initial trials June 2012
Example results
EMI at the Woburn site – conductivity over 50cm depth
5-Jun-2012
We aim to develop a new methods of measuring rootfunction that is rapid, non-destructive and accurate
• EMI and ERT data will be compared with data fromburied soil moisture meters, soil sampling at variousdepths, root depth measurement with transparentrhizotrons and the emerging qPCR approach tomeasuring root DNA concentration in soil.
• Data from these invasive approaches will be used tovalidate and refine as necessary the EMI protocol.
Part of project funded by BBSRC, CRIC project: Phenotyping root function inwheat
Finally: Future Project